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Volt
The volt (symbol: V') is the for , , and . The volt is named in honor of the Italian physicist (1745–1827), who invented the , possibly the first chemical . Definition A single volt is defined as the ''difference in across a when an of one dissipates one of .BIPM SI Brochure: Appendix 1, p. 144 It is also equal to the potential difference between two parallel, infinite planes spaced 1 apart that create an of 1 per . Additionally, it is the potential difference between two points that will impart one of per of charge that passes through it. It can be expressed in terms of SI units as follows: : \mbox{V} = \mbox{A} \cdot \Omega= \dfrac{\mbox{W}}{\mbox{A}} = \sqrt{\mbox{W} \cdot \Omega} = \dfrac{\mbox{J}}{\mbox{A} \cdot \mbox{s}} = \dfrac{\mbox{N} \cdot \mbox{m} }{\mbox{A} \cdot \mbox{s}} = \dfrac{\mbox{kg} \cdot \mbox{m}^2}{\mbox{C} \cdot \mbox{s}^2} = \dfrac{\mbox{N} \cdot \mbox{m}} {\mbox{C}} = \dfrac{\mbox{J}}{\mbox{C}} It can also be written using only the SI base units m, kg, s, and A as: : \mbox{V} = \dfrac{\mbox{kg} \cdot \mbox{m}^2}{\mbox{A} \cdot \mbox{s}^{3}} Josephson junction definition Between 1990 and 1997, the volt was calibrated using the for exact voltage-to-frequency conversion, combined with cesium-133 time reference, as decided by the 18th . The following value for the is used: :K''{J-90} = 2 /'' = 0.4835979 GHz/µV. This is typically used with an array of several thousand or tens of thousands of , excited by microwave signals between 10 and 80 GHz (depending on the array design). Empirically, several experiments have shown that the method is independent of device design, material, measurement setup, etc., and no correction terms are required in a practical implementation. However, as of July 2007, this is not the official definition of Volt.http://www.bipm.org/en/si/si_brochure/chapter2/2-1/ Water flow analogy In the '' sometimes used to explain electric circuits by comparing them to water-filled pipes, difference is likened to water difference–the difference determines how quickly the electrons will travel through the circuit. Current (in amperes), in the same analogy, is a measure of the volume of water that flows past a given point per unit time ( ). The flow rate is determined by the width of the pipe (analogous to ), and the pressure difference between the front end of the pipe and the exit (analogous to ). The analogy extends to power dissipation: the power given up by the water flow is equal to flow rate times pressure, just as the power dissipated in a resistor is equal to current times the voltage drop across the resistor. The relationship between voltage and current (in ohmic devices) is defined by . Common voltages can be used to measure the voltage between two positions.}} Nominal voltages of familiar sources: * : around −75 mVBullock, Orkand, and Grinnell, pp. 150–151; Junge, pp. 89–90; Schmidt-Nielsen, p. 484 * Single-cell, rechargeable or battery: 1.2 V * : 1.355 V * Single-cell, non-rechargeable (e.g., ): 1.5 V * rechargeable battery: 3.3 V * rechargeable battery: 3.75 V (see ) * / (TTL) power supply: 5 V * : 9 V * electrical system: nominal 12 V, about 11.8 V discharged, 12.8 V charged, and 13.8–14.4 V while charging (vehicle running). * Household : 230 V in Europe, Asia and Africa, 120 V RMS in North America, 100 V RMS in Japan (see ) * s/ : 24 V DC * : 600–750 V (see ) * High-speed train overhead power lines: , but see the and for exceptions. * High-voltage lines: 110 kV RMS and up (1.15 MV RMS was the record as of 2005 * : Varies greatly, often around 100 MV. Note: Where RMS ( ) is stated above, the peak voltage is \sqrt{2} times greater than the RMS voltage for a signal centered around zero voltage. History of the volt In 1800, as the result of a professional disagreement over the galvanic response advocated by , developed the so-called , a forerunner of the , which produced a steady electric current. Volta had determined that the most effective pair of dissimilar metals to produce electricity is and . In the 1880s, the International Electrical Congress, now the (IEC), approved the volt as the unit for electromotive force. At that time, the volt was defined as the potential difference what is nowadays called the "voltage (difference)" across a conductor when a current of one dissipates one of power. The international volt was defined in 1893 as 1/1.434 of the of a . This definition was abandoned in 1908 in favor of a definition based on the international and international ampere until the entire set of "reproducible units" was abandoned in 1948. Prior to the development of the Josephson junction voltage standard, the volt was maintained in national laboratories using specially constructed batteries called ''' . The United States used a design called the from 1905 to 1972. See also * * * * * * * for unit prefixes * * * References External links * * Google Video Lecture about Electricity and Volt by Richard A. Muller * Electricity Electric Field Strength Tool Category:SI derived units Category:Units of electrical potential